1. Field of the Invention
The present invention relates to a high voltage power supply used in a laser printer or a multifunction peripheral. More particularly, the present invention relates to a high voltage power supply that is capable of preventing the generation of an abnormally high voltage, which can deteriorate image quality when applied to a surface of a photoconductive drum.
2. Description of the Related Art
In a multifunction peripheral having a printing function such as a laser printer, a laser beam is radiated onto an electrostatically charged photoconductive drum to produce latent characters or images to be printed on a sheet of paper. The latent characters or images are developed onto printing paper using toner. The developed characters or images are fixed onto printing paper by a fixing process. The multifunction peripheral or laser printer requires a high voltage to achieve the above-described functions. To this end, the multifunction peripheral or laser printer includes a high voltage power supply.
FIG. 1 is a block diagram of a conventional high voltage power supply. The high voltage power supply in FIG. 1 includes a pulse width modulation (PWM) signal input unit 100, a switching drive control unit 110, a switching unit 120, a boosting and rectifying unit 130, an offset voltage supplying unit 140, and a voltage branching unit 150. The PWM signal input unit 100 receives an enable signal IN1 from a central processing unit (CPU, not shown), which is indicative of whether to output a high voltage. The switching drive control unit 110 outputs a switching drive control signal by comparing a reference signal, which is the enable signal IN1 via the PWM signal input unit 100, with an output signal. The switching unit 120 then generates a voltage for a primary coil of a fly-back transformer (FBT) in the boosting and rectifying unit 130 according to the switching drive control signal. The fly-back transformer (not shown) of the boosting and rectifying unit 130 boosts the voltage generated at the primary coil (not shown) and outputs the boosted voltage to an output terminal of a secondary coil (not shown). Further, the boosting and rectifying unit 130 rectifies the boosted voltage and outputs an output signal OUT1. The offset voltage supplying unit 140 supplies an offset voltage to the output signal OUT1, which is fed back to the switching drive control unit 110 via the voltage branching unit 150. The voltage branching unit 150 branches off the voltage of the output signal to feed it back to the switching drive control unit 110 such that the output signal can be compared with the reference signal.
FIG. 2 is an exemplary circuit diagram of the switching drive control unit 110. The switching drive control unit 110 is configured as a proportional integral (PI) controller and compares a composite signal IN3 (the output signal of the boosting and rectifying unit 130 and the offset voltage supplied from the offset voltage supplying unit 140), with a reference signal IN2 input from the PWM signal input unit 100, and outputs a drive control signal OUT2. That is, the switching drive control unit 110 senses a difference between the composite signal IN3 and the reference signal IN2, and outputs a drive control signal to the switching unit 120 based on the difference. The switching drive control unit 110 increases the voltage level of the output drive control signal if the input reference signal IN2 has a lower level than the composite signal IN3. The switching drive control unit 110 decreases the voltage level of the output drive control signal if the reference signal IN2 has a higher level than the composite signal IN3.
When power is first turned on, a PWM signal transitions from a low logic state to a high logic state, and is maintained in the high logic state in a standby mode. At this time, a high voltage output is not desired. However, since the composite signal IN3 comprises the offset voltage, the offset voltage reaches a non-inverting terminal (+) of an operational amplifier OP1 earlier than the reference signal IN2 input from the PWM signal input unit reaches an inverting terminal (−) of the operational amplifier OP1 when power is turned on. As a result, the output signal of the operational amplifier OP1 has a high initial voltage level just after power is turned on. The waveforms of the two input signals and the output signal of the operational amplifier OP1 when power is turned on are illustrated in FIG. 3. As illustrated in FIG. 3, the offset voltage is rapidly applied from the offset voltage supplying unit 140 to the non-inverting terminal (+) of the operational amplifier OP1 such that an output signal OUT1 having an undesirably high voltage level is output. However, when an abnormally high voltage signal is applied to a surface of the photoconductive drum when power is turned on, the quality of the desired images deteriorates and the printing operation becomes unstable.
Accordingly, a need exists for a high voltage power supply and method of use that is capable of preventing the generation and application of an abnormally high voltage to a surface of a photoconductive drum when power is turned on.